Four (4) main types of automotive power trains, commonly known as full-time rear wheel drive systems, full-time front wheel drive systems, part-time four wheel drive systems, and all wheel drive systems are known. All-wheel drive systems are gaining popularity for their ability to deliver driving traction to all four vehicle wheels with undiminished vehicle speed. All-wheel drive systems, however, often suffer from the inability to vary the amount of torque delivered between the front and rear drive axles. Common systems have a single speed with continuous all-wheel drive. Common all-wheel drive systems therefore have limited or no capability to maximize drive torque for “off-road”, snow, or similar driving conditions when greater drive torque at slower vehicle speeds is desirable.
All-wheel drive systems are known which use a power take-off unit (PTU) to distribute torque between the front and rear drive axles of a vehicle. Torque is normally distributed in a predetermined percentage, such as 40 percent to the front drive axles and 60 percent to the rear drive axles. Known PTU's do not provide the capability to both vary the amount of torque delivered between the front and rear drive axles and to provide different vehicle operational speed ranges. This includes the capability to provide, for example, a varying torque to the different drive axles and a low speed range for low traction driving conditions.